Motors - another concept for FRC
 

When a mentor (Shaun) on my team mentioned an idea some time ago, I just could not get it out of my head. It was radical and interesting but do-able so I thought I'd post it here for your opinion.

We are given a set of roughly 10 motors every year that are legal to use. According to the rules, for most motors, we can only use what is given to us in the kit and in the same quantity that they are given. What if the rules were written as such: We are given the names of 7 (arbitrary number) motors legal for an FRC competition. We are allowed to use as many of each type of motor as long as the total number of motors is less than 10 (again, arbitrary number). There are pros and cons to this concept for the robot design and inspection. I'll let you point them out.

It's just a concept.. I am not advocating anything.. :rolleyes:

Re: Motors - another concept for FRC
 

Battery, Maximum Weight, Maximum Size... no other restrictions. :)

-John

Re: Motors - another concept for FRC
 

Maximum power, safety.

(I got that idea from another competition we've participated in)

Re: Motors - another concept for FRC
 

Its a fun idea, but not really practical in a design challenge.

+.02

Re: Motors - another concept for FRC
 

it's practical.

Re: Motors - another concept for FRC
 

I predict a large demand for CIM motors, as they seem to be the most popular.

Re: Motors - another concept for FRC
 

I gotta agree with John here. You would think that removing constraints makes things easier but it doesn't. In fact it would open up entirely new decisions to make thus further complicating the design. I would add another restriction, maximum cost. (And I assume safety is factored in too)

Akash, the reason this isn't practical is because we simply don't have the TIME to research the optimum component for each aspect of the game. By constraining us to 10 motors the GDC is restricting our decision to help us, instead of choosing from hundreds of motors we can only choose from 10. And frankly many of them only make sense in certain situations which further simplifies the decision.

A lot of times people complain about wanting a more powerful motor, I for one am actually quite happy about the motors we have. It would be interesting to open up the motor selection a little more, add some choices that make motors exclusive, ie you can have X CIMs or Y of a motor with slightly different characteristics (lower torque, less current draw etc) Perhaps add in an off the shelf linear actuator. Perhaps allow extra weight for certain drive systems, if you build a walker (a much more mechanically complex system) you get an extra X pounds and may use Y more motors of varying type. Decisions like this might encourage teams to think outside the box. While the 6wd is a nice drive system should we encourage people to keep doing what they have always done because they are comfortable or should we encourage them to try new things?

Just my $.02

Re: Motors - another concept for FRC
 

If the manual said "You may use nothing but CIM motors and Globe motors" I'd send the GDC members each a cookie. I'd send Kate Pilotte two.

Re: Motors - another concept for FRC
 

No FP's? I'd like CIM, FP and Globe.

Re: Motors - another concept for FRC
 

I wouldn't be totally opposed to opening up motor selection if it included some restrictions: specifically a limit to the retail cost of a motor (preferably around $30, a little more than a CIM). I would hate to see teams with fewer resources outclassed because a team could buy a $200 motor.

That being said, I really enjoy allocating motors every year. I find its a great challenge for both myself and my students. I suppose this part of the challenge is rather unique, and in turn makes FIRST a more unique experience.

Re: Motors - another concept for FRC
 

If you could have a CIM instead would you really use a FP?
To me that is a NO brainer.

Re: Motors - another concept for FRC
 

I would be a little hesitant about it, but I can imagine using multiple FPs instead of a CIM for higher up manipulators that still need some power. A better application is probably for multiple smaller manipulators, where the weight of CIMs would be killer.

Re: Motors - another concept for FRC
 

John: The CIM weighs a lot more. That might be enough to put an FP in where a CIM could go. Just one of those accursed design tradeoffs that come with any design problem. And yes, I might use an FP instead of a CIM even if I could use a CIM. It depends on the application.

Back to what I was originally going to say...

I compete in the SAE Aero Design competition. There are three classes of planes, each with different power restrictions. Regular says "Thou SHALT use engine X with muffler Y, unmodified." Advanced says simply, "Your total stamped engine displacement must be less than or equal to Z cubic inches. Reciprocating engines only, please."

But the most interesting is the Micro class. There are no engine restrictions, other than that it has to be internal combustion (reciprocating) or electric. This opens up a wide variety; and my team has done both. Gas was our choice the first year, because we knew what we were doing. The next year, we chose a pair of electric motors instead. Different planes, same goal, different power, different results.

The plethora of options is one thing; it can make decisions harder. On the other hand, having few/no options makes them easier, but not necessarily. FIRST has struck a balance: You can use this number/type of motor, here are the motors.

Re: Motors - another concept for FRC
 

And bring back the ol' van door motor from '04 era... that thing was a beast!

Re: Motors - another concept for FRC
 

I really like the idea of say, any combination of CIM, FP, Globe, and Van Door motors (or any other 12V VDC motor that costs less than say, $30 or $50), as long as the total is less than 10 motors. The only other restriction I'd maybe put is a maximum total Wattage of motors used to power the drive train. Some teams would forget or ignore whether their drive trains were torque or traction limited, and would assume that more motors is always more traction, and you'd see 10-CIM drive trains (like how we saw 4-CIM drive trains this year). This would be fine on a concrete floor, but on carpet the amount of available power there would destroy the carpet in a pushing match in seconds. (Although this scenario could be covered under a "Don't Damage the Field" rule, as if a team had a 10-CIM drive train but also had traction-control code to prevent the drivetrain wheels from slipping and thus potentially damaging the floor, they wouldn't ever actually damage the floor).

It's kind of like the idea that weight limits are unnecessary for Vex robots. If you want to build a 50-pound Vex robot, you'll pay for it in speed and battery life. You want to build an eight degree of freedom arm all powered by CIM motors? Enjoy your thirty second battery life.

As we finally saw this past season, FIRST decided to treat teams to a little more responsibility by eliminating the "No Tape Rule". Now, we could finally use duct tape if we wanted to. And you know what? The quality of the robots ended up just fine, and having the ability to use duct tape for emergency repairs was a great asset this year for many teams.

But at the same time, we still need some rules and restrictions. If we eliminated all restrictions and basically said "use up to $3000 to build anything you want to play this game", we'd end up with a lot of teams getting the Deer-in-headlights syndrome, staring at a blank sheet of paper with no idea on where to begin. Restrictions, just like engineering task specifications in the real world, help narrow down the problem effectively enough to make it easier to solve.

Re: Motors - another concept for FRC
 

Unless I'm building a high-speed, low-torque, application with a low chance of stalling the motor (say, a "shooting wheel" as used in 2006 and 2009 by many teams), I definitely agree with JVN. CIMs are simply much more reliable and a better fit for most FRC applications than a FP.

Also, I <3 globes. My favorite motor in the kit for non-drive applications, without a doubt.

Re: Motors - another concept for FRC
 

Let’s face it, just about everything on an FRC robot is driven by either a motor or a pneumatic solenoid. OK, so there are some uses for springs and servos and such but the bulk of everything that FRC robots “do” are ultimately actuated by either a motor or a pneumatic solenoid. I welcomed the changes this past year in opening up the choices we had for pneumatic solenoids. I think FIRST is on the right track. Now maybe it is time to look at the motors.

Many teams, especially rookies, don’t have the resources (tooling, experience, funds, whatever) to develop custom gearboxes. The FP motors are a fairly standard 550 style “can” motor thus there are several off the shelf options for gearboxes that will turn the high speed, low torque FP motors into something with a more usable speed and torque output and just as importantly something with a more reasonable output shaft format such as a ½” keyed shaft (or whatever). Likewise, the CIM motors have several COTS gearbox options from companies like AndyMark and BaneBots. We even get usable CIM motor gearboxes in the KoP. That’s why the majority of teams, again especially rookies, use the CIM motors and the KoP “CIM gearbox of the year” (aside from the BaneBots planetary debacle of 2007).

Yes, we get a bunch of other odd ball motors in the KoP but most of them have such funky output shafts that they are just a royal PITA to use. Heck, most of the time we don’t get the special electrical connectors required to even use some of the window motors and such so we end up hacking some kind of unreliable soldered up connection. The Globe motor is sort of in between and isn’t too bad to deal with. We can talk all we want about optimal torque curves and choosing the most efficient motor for the job, etc. but when it comes down to it a lot of teams look at the pile of motors they are given to work with and say “these are ones we can do something with given our resources so lets design around these” and “all these other funky ones are kind of useless to us even if they would be great for an arm or wharever we can't make anything that uses that wierd output”.

There is a reason AndyMark and BaneBots sell such nice options for gearboxes for the FP (and other 500/550 style can motors) and for the CIM motors and not the window motors, etc. The FP/CIMs are a known quantity. The COTS gearboxes adapt the speed and torque of a FP/CIM to something more reasonable and just as importantly something we can easily stick a hub and sprocket or pulley or wheel or whatever on. If you are a team with a drill press and a miter saw it can be quite difficult to make something that reliably uses some of the other odd ball motor outputs. That is one reason I feel the designs are being too limited by the motors we are given to use. If more/all of the motors in the KoP had more standard/common/usable output interfaces like say a ½” keyed shaft (or whatever) then there would be many more off the shelf options as far as gears, sprockets, etc. that teams with limited resources could just acquire and use. Then many of the limitations on robot designs and capabilities would be reduced and the whole level of play would be raised.

I know a lot of the motors with the funky outputs like the window motors, van-door motors, etc. are probably donated automotive industry surplus and that’s fine. If FIRST gets them for free or real cheap then by all means throw them in the KoP. But if we all get 10-12 motors each year but only half of them are really usable by a large number of teams then designs and capabilities are being limited for a lot of those teams when compared to the teams that do have the resources to use more of the odd ball motors. In effect some teams get 10-12 motors in their kit while others get 4-6. For these reasons I would love to somehow see more options to use some kind of motors/gearboxes with outputs that are a bit easier to work with for ALL teams.

Re: Motors - another concept for FRC
 

Did you account for the weight you are going to add to get the FP motor to an equivalent spec of a CIM motor? Unless your application requires ~15000 rpm at w/e the output torque of the FP is theres going to be some weight gain there.

Sure there will be weight for the CIM too because not too many applications are goign to use the CIM at a free speed of ~5000 rpm, but to simply get the FP to be similar in speed and torque as a CIM it will require a signifigant amt. of weight.

I agree with JVN on this one, given the choice of FP or CIM I would go CIM everytime, regardless of any kind of weight gain. The reliability factor alone is worth it.

Brando

Re: Motors - another concept for FRC
 

As I recall, the last time 330 smoked an FP was when we had the 6V ones given in the KOP instead of the 12V ones, if then. The time before then was deliberate... Reliability, if the system is designed right, is just as good as with a CIM.

Sure, you need a gearbox with an FP. A CIM is about 3 pounds, IIRC, and an FP is about one pound. If your gearbox (say, an AM or BB planetary) is less than 2 pounds, you're lighter for close to equivalent functionality.

Re: Motors - another concept for FRC
 

If we want to open up some of these choices we already have the materials in previous KOPs...

By limiting the power to one battery.... and also limiting the output to a single 100 amp breaker.... we could limit power.... (we already do this....)

We could also say... any motor... BUT you are only allowed
4 motors on 40 Amp breakers,, 4 motors on 30 Amp breakers and 4 motors on 20 Amp breakers but not to exceed 10 motors...

This along with the weight limit for the entire robot would really limit what a robot could do....and level the playing field

Judicious use of the power that you have would be necessary... (not unlike what we have to do now....)

I think that this would lead to a very interesting engineering experience involving the actual calculation of power loads and drains...

Many of the teams already do this but it would be much more real world.

I also really like the idea of giving three motors...CIM,FP, GLOBE in unlimited quantities but with the above restrictions on power use.

If a team chose to use 10 CIMS they would have some serious issues with Power consumption to deal with... (not to mention weight..)

Interesting proposal....

Re: Motors - another concept for FRC
 

Richard's first rule of motor selection: you pay for torque.

How you pay varies somewhat by application. It might be money, weight, space claim, service life, etc.; or combinations that include some or all of these 'costs'.

Richard's second rule: choose a motor that can take the heat.

How much heat must your motor take? Make sure you are confident of the answer to that question as early in the design process as you can, hopefully before your preliminary choice becomes so entrenched that changing it will be a major upset (or worse).

Quite often, FRC robot designers (and designers of many other things) don't really find out how much heat that is until they encounter a rare (but usually forseeable) overload. Think about how someone could overload your system.

----------

Back to the OP's main topic: As a motor designer I'd love to see more varieties of my favorite technology showcased; however, as a FIRST mentor and volunteer, I know that limiting motor selection to a short list is consistent with the system-thinking emphasis and tight schedule constraints that make FRC an inspirational challenge.

Re: Motors - another concept for FRC
 

A very smart man once said...

Spoken like a true genius.

FIRST spends a lot of time finding all of these types of items for the kits......open up the rules and put the burden on the teams!

Re: Motors - another concept for FRC
 

No worries about picking from hundreds - you must pick from seven, explicitly specified.

One issue is that many of the motors we are given are not available in huge quantities on the open market. If every team decided to use 5 of the LH window motors, I think the Earth would be hard-pressed to supply them.

One alternative would be to supply some motors, and give teams the option that they can use "x quantity of these other, not-supplied motors if you want".

On a related topic: I sure would like to see an off-the-shelf linear actuator (like those used to turn satellite dishes) become available. Slow but powerful.

Re: Motors - another concept for FRC
 

Battery, Breaker, Max Weight, Max Size.

Re: Motors - another concept for FRC
 

I like the idea that teams can use as many of each type of KoP motor as they want, as long as its under a certain number. If you wanted to limit it even more, you could put a limit on the total power (wattage) that all the motors together might be less than.

Re: Motors - another concept for FRC
 

Not to take a side on the FP vs. CIM debate, but an FP + AM Planetary to get it around the same output speed as the CIM is actually about 1 lb less than a CIM. We looked into this as a weight savings option for 2009, but found the weight in other areas. Just want to pepper some facts into our daily discussion.

Re: Motors - another concept for FRC
 

I think the absence of a full and structured set of rules would be the failure of FRC.

Re: Motors - another concept for FRC
 

David, I think the expansion of the MI district system will be the failure of FRC.

Note: The above statement is not repeat not my real view. I am making it to make a point.

Anyone can say anything they like, but I'd rather have some reasons. I don't really care if it's just plain, "I don't like it", as long as it's a reason. Especially if it's a reasonable reason or brings up something I didn't see.

I can see where he's coming from. FRC started with a very restrictive set of rules. KOP, $X00 from SPI, Y, Z, A, B, C, in various amounts from Home Depot, here's the game, go build us a robot. Then they relaxed the rules, in part because suppliers were getting overwhelmed. They're very open now. If they're opened further, nobody really knows what's going to happen.

But I don't think FRC would fail. It could get very interesting, though, as teams face more difficult decisions. It would also depend on the manner of the expansion--X types in Y amounts, anything under $Z, A power, 12V system, or any other type of expansion you can imagine. But it probably wouldn't fail. It might even become stronger.

Re: Motors - another concept for FRC
 

The FP + Banebot 42 mm planetary (currently in P60 form) is even lighter and more compact, I believe. This has been a reliable combo for us the past few years. The RS-550 has also been a reliable workhorse when coupled to this gearbox. In many, but not all, cases, I'd still prefer this option over a CIM + reduction of choice.

Re: Motors - another concept for FRC
 

I can see the FP being lighter as you have pointed out here, and I really dont have a strong preference either way. I simply like the reliability of a CIM over that of a FP, so much so that in most (but not all cases) i'd be willing to sacrifice that pound of weight.

Re: Motors - another concept for FRC
 

2337 did just that this year. I will try to convince someone from that team to post the experience.

Re: Motors - another concept for FRC
 

2815 did the same thing this year for their drivetrain. Not so much for weight savings as the fact that all four CIMs were tied up in our conveyor and shooter drum system. No issues all season (nor in its several off-season demos) driving on a slick floor, though we did keep cans of compressed air in the pits. I don't think we'd do it like this in a carpet game, but it worked great for the task at hand.

Re: Motors - another concept for FRC
 

i'd take the great weight for if just to get the Reliability of a CIM
fp's are vented, have really bad bushings, and spin at 15,000rpm it's not a good combo for what we do to them

Re: Motors - another concept for FRC
 

Sorry for dragging this up again -- I want to (try to) clarify what I said in an earlier post, using some data.The 12 Volt CIM and FP motors available from AndyMark have different parameters, consistent with their different sizes, weights, and costs:

CIM: 2.22 N-m stall torque, 115 Ampere stall current, 1280 g mass, $28 price
FP: 0.45 N-m stall torque, 70 Ampere stall current, 272 g mass, $13 price

To get about the same torque that is available from a CIM, others have pointed out that the FP can be combined with a 3.67:1 gearbox that has 409 g mass and $98 price.

Rated voltage (V), stall torque (ST), and stall current (SC) can be conveniently combined to give a figure-of-merit that is often called the 'motor constant' and is defined by:

kM = ST / sqrt (V*SC), which has units of torque over the square root of power.

When we use Newton-meter as the torque unit, kM is expressed in Newton-meter per root Watt. Its physical significance lies in the following simple expression for Joule (i.e., I^2 * R) losses in the motor for a given torque:

Pj = (torque / kM)^2

As an example, let's calculate Joule losses in the CIM and FP motors when each is used to drive 35 lbf traction load on a 6" diameter wheel through a 12.75:1 Toughbox. (Note that this loading, while extreme, has very likely been seen in many FRC situations over the past few competition seasons. Maybe not so often in 2009, when traction on the playing surface was limited.)

The specified load corresponds to 105 lbf-in torque at the wheel, and to 8.2 lbf-in torque at the Toughbox input. (Note: for simplicity I am neglecting mechanical losses in gearboxes; this would make conclusions regarding absolute rates of motor heating somewhat optimistic, but will not impact conclusions regarding relative rates of motor heating.)

So in this example the CIM torque is 0.931 Newton-meter (42% of its stall torque) and the FP torque is 0.931 / 3.67 = 0.254 Newton-meter (56% of its stall torque).

Now let's calculate kM for each motor:

CIM: kM = 2.22 / sqrt(12*115) = 0.0598 Newton-meter per root Watt
FP: kM = 0.45 / sqrt(12*70) = 0.0155 Newton-meter per root Watt

From these figures we calculate the Joule losses in each motor at their respective torque loads:

CIM: Pj = (0.931 / 0.0598)^2 = 242 Watt
FP: Pj = (0.254 / 0.0155)^2 = 267 Watt

These losses will cause the motors to heat up. How fast will the each motor's temperature rise? This brings us to Richard's second rule:

Rate of temperature rise is roughly proportional to the ratio of the motor's power losses to its mass. (Note the proportionality will not be the same for motors made of dissimilar materials; however, relatively conventional motors like the ones used in this example are made mostly of steel and copper, so the approximation is valid.) The loss-to-mass ratios in this example are:

CIM: 242 / 1.28 = 189 Watt/kg
FP: 267 / 0.272 = 979 Watt/kg

So the FP will exhibit about 979/189 = 5.17 times the rate of temperature rise. Put another way, if the CIM can take a 35 lbf wheel traction load for 30 seconds before overheating, then the FP can take the same load for about 6 seconds.